Interior angles can be calculated uing foreshots and backshots. The sum of the interior angles
Use the following table as an example.
|
STA |
FS |
BS |
|
4 |
|
|
|
|
20 |
200 |
|
1 |
|
|
|
|
130 |
310 |
|
2 |
|
|
|
|
205 |
27 |
|
3 |
|
|
|
|
345 |
165 |
|
4 |
|
|
|
|
20 |
200 |
|
1 |
|
|
The foreshot azimuth from station 1 to 2 is 130°. The backshot azimuth from 4 to 1 is 200°. The angle 4-1-2 is equal to the difference of these azimuths:
200°-130° = 70°
Likewise, 2-3-4 is:
27°-345° = -318°
but -318° doesn't make much sense, so subtract it from 360°:
360°-318° = 42°
|
STA |
FS |
BS |
IA |
|
4 |
|
|
|
|
|
20 |
200 |
|
|
1 |
|
|
70 |
|
|
130 |
310 |
|
|
2 |
|
|
105 |
|
|
205 |
27 |
|
|
3 |
|
|
42 |
|
|
345 |
165 |
|
|
4 |
|
|
145 |
|
|
20 |
200 |
|
|
1 |
|
|
|
And the sum of the measured interior angles is 362.